A transparent electrode has become an indispensable component for a display device such as an organic EL (electroluminescence) display, a liquid crystal display, etc., a touch panel used for a tablet-type portable terminal such as a smartphone, and used for other various input devices, etc., a solar cell, and the like.
Conventionally, ITO (Indium Tin Oxide) has been used for transparent conductive films used for such transparent electrodes. However, indium used for ITO is a rare metal, and recently, stabilizing the supply and the price of indium has become an issue. Also, for the formation of ITO films, a sputtering method, a vapor-deposition method, and the like are used, and thus, a vacuum production apparatus is required and the production takes a long time, resulting in the higher cost. Further, a crack may be easily generated in ITO due to a physical stress such as bending and ITO can be easily broken. Therefore, applying ITO to a flexible substrate is difficult. Accordingly, alternative materials of ITO capable of overcoming these drawbacks have been searched for.
Among “alternative materials of ITO”, as materials which do not require the use of a vacuum production apparatus, and which can be used for forming films by coating, conductive materials, for example, (i) polymer-based conductive materials such as poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonic acid) (PEDOT: PSS) (for example, refer to Patent Document 1), conductive materials containing a nanostructured conductive component such as (ii) metal nanowires (for example, refer to Patent Document 2 and Non-patent Document 1), and (iii) carbon nanotubes (for example, refer to Patent Document 3), have been reported.
Among these, (ii) conductive materials containing metal nanowires are reported to have a low surface resistance and a high light transmittance (for example, refer to Patent Document 2 and Non-patent Document 1), and further, have flexibility, and thus, are suitable as “alternative materials of ITO”.
Here, for example, in case of an organic EL element, an ultrathin film of an organic compound is formed on an electrode. Thus, when a transparent electrode having a low surface smoothness is used, functions of the organic EL element are decreased. Therefore, a superior surface smoothness is required for a transparent electrode.
When a transparent electrode is formed using the above-mentioned conductive material containing metal nanowires, in order to increase the surface flatness of the transparent electrode, there is a proposed method of forming a conductive layer with metal nanowires on a support having high flatness, and of transferring the formed conductive layer to another support (Patent Document 4). However, adjusting the balance between the adhesiveness and releasability of the adhesive agent used for transfer with the support and the conductive layer is difficult, and thus, perfect transfer is difficult. Further, this method has a large number of steps including coating an adhesive layer, curing, laminating support films, and releasing, which leads to the increase in cost.
Patent Document 5 discloses a method for producing a transparent electrode by a step of forming a first conductive layer by coating a liquid containing metal nanowires and a binder, a step of cross-linking or curing the binder, a step of forming a second conductive layer by coating an aqueous dispersion containing a conductive polymer and a non-conductive polymer on the first conductive layer, and a step of pattern printing nanowire removing liquid and washing with water. In this method, in order to ensure the surface flatness, two layers are formed, which leads to the increase of the production steps, and the disadvantage in regards to productivity.